Galileo did not make his first contribution to astronomy until 1604, when a supernova, or a massive explosion of a star, suddenly appeared in the night sky. Galileo reasoned that this object was farther away than the planets and pointed out that this violent event in the sky meant that the "perfect and unchanging heavens" that Aristotle had claimed surrounded Earth were not unchanging after all.
JUPITER
On 7 January 1610, Galileo observed with his telescope what he described at the time as "three fixed stars, totally invisible by their smallness", all close to Jupiter, and lying on a straight line through it. Observations on subsequent nights showed that the positions of these "stars" relative to Jupiter were changing in a way that would have been inexplicable if they had really been fixed stars. On 10 January, Galileo noted that one of them had disappeared, an observation which he attributed to its being hidden behind Jupiter. Within a few days, he concluded that they were orbiting Jupiter / He had discovered three of Jupiter's four largest satellites (moons). He discovered the fourth on 13 January. Galileo named the group of four the Medicean stars, in honor of his future patron, Cosimo II de' Medici, Grand Duke of Tuscany, and Cosimo's three brothers. Later astronomers, however, renamed them Galilean satellites in honor of their discoverer. These satellites are now called Io, Europa, Ganymede, and Callisto.
KEPLER'S SUPERNOVA
According to Walusinsky, Galileo's fame as an astronomer dates to his observation and discussion of Kepler's supernova in 1604. Since this new star displayed no detectable diurnal parallax, Galileo concluded that it was a distant star, and therefore disproved the Aristotelian belief in the immutability of the heavens. His public advocacy of this view met with strong opposition.
Galileo was one of the first Europeans to observe sunspots, although Kepler had unwittingly observed one in 1607, but mistook it for a transit of Mercury. He also reinterpreted a sunspot observation from the time of Charlemagne, which formerly had been attributed (impossibly) to a transit of Mercury. The very existence of sunspots showed another difficulty with the unchanging perfection of the heavens as posited in orthodox Aristotelian celestial physics. And the annual variations in sunspots' motions, discovered by Francesco Sizzi and others in 1612–1613, provided a powerful argument against both the Ptolemaic system and the geoheliocentric system of Tycho Brahe. A dispute over priority in the discovery of sunspots, and in their interpretation, led Galileo to a long and bitter feud with the Jesuit Christoph Scheiner; in fact, there is little doubt that both of them were beaten by David Fabricius and his son Johannes. Scheiner quickly adopted Kepler's 1615 proposal of the modern telescope design, which gave larger magnification at the cost of inverted images; Galileo apparently never changed to Kepler's design.
Galileo did not make his first contribution to astronomy until 1604, when a supernova, or a massive explosion of a star, suddenly appeared in the night sky. Galileo reasoned that this object was farther away than the planets and pointed out that this violent event in the sky meant that the "perfect and unchanging heavens" that Aristotle had claimed surrounded Earth were not unchanging after all.
JUPITER
On 7 January 1610, Galileo observed with his telescope what he described at the time as "three fixed stars, totally invisible by their smallness", all close to Jupiter, and lying on a straight line through it. Observations on subsequent nights showed that the positions of these "stars" relative to Jupiter were changing in a way that would have been inexplicable if they had really been fixed stars. On 10 January, Galileo noted that one of them had disappeared, an observation which he attributed to its being hidden behind Jupiter. Within a few days, he concluded that they were orbiting Jupiter / He had discovered three of Jupiter's four largest satellites (moons). He discovered the fourth on 13 January. Galileo named the group of four the Medicean stars, in honor of his future patron, Cosimo II de' Medici, Grand Duke of Tuscany, and Cosimo's three brothers. Later astronomers, however, renamed them Galilean satellites in honor of their discoverer. These satellites are now called Io, Europa, Ganymede, and Callisto.
KEPLER'S SUPERNOVA
According to Walusinsky, Galileo's fame as an astronomer dates to his observation and discussion of Kepler's supernova in 1604. Since this new star displayed no detectable diurnal parallax, Galileo concluded that it was a distant star, and therefore disproved the Aristotelian belief in the immutability of the heavens. His public advocacy of this view met with strong opposition.
Galileo was one of the first Europeans to observe sunspots, although Kepler had unwittingly observed one in 1607, but mistook it for a transit of Mercury. He also reinterpreted a sunspot observation from the time of Charlemagne, which formerly had been attributed (impossibly) to a transit of Mercury. The very existence of sunspots showed another difficulty with the unchanging perfection of the heavens as posited in orthodox Aristotelian celestial physics. And the annual variations in sunspots' motions, discovered by Francesco Sizzi and others in 1612–1613, provided a powerful argument against both the Ptolemaic system and the geoheliocentric system of Tycho Brahe. A dispute over priority in the discovery of sunspots, and in their interpretation, led Galileo to a long and bitter feud with the Jesuit Christoph Scheiner; in fact, there is little doubt that both of them were beaten by David Fabricius and his son Johannes. Scheiner quickly adopted Kepler's 1615 proposal of the modern telescope design, which gave larger magnification at the cost of inverted images; Galileo apparently never changed to Kepler's design.
(http://en.wikipedia.org/wiki/Galileo_galilei#Jupiter)